Interfacial Impregnation Chemistry in the Synthesis of Nickel Catalysts Supported on Titania

Abstract

The interfacial chemistry of the impregnation step involved in the preparation of nickel catalysts supported on titania is presented. Several methodologies based on deposition data, pH measurements, potentiometric mass titrations, and microelectrophoresis have been used in conjunction with diffuse reflectance UV/Vis/NIR spectroscopy, simulations, and semiempirical quantum chemical calculations. Three mononuclear inner-sphere complexes were formed at the compact layer of the "titania/electrolyte solution" interface: A monosubstituted, dihydrolyzed complex above a terminal oxo group, a disubstituted, dihydrolyzed complex above two terminal adjacent oxo groups, and a disubstituted, nonhydrolyzed complex above one terminal and one bridging adjacent oxo groups. The monosubstituted, dihydrolyzed complex predominates. The contribution of the disubstituted configurations is also important at very low Ni(II) surface concentration, but it decreases as the Ni(II) surface concentration increases. In addition, bi- and trinuclear inner-sphere complexes were formed. The receptor site involves one bridging and two terminal oxo groups in the first case and two bridging and three terminal oxo groups in the second case. The relative surface concentrations of these configurations increase initially with Ni(II) surface concentration and then remain practically constant. The understanding of these interfacial processes at a molecular level is very important to shift the catalytic synthesis from an art to a science as well as to obtain strict control of the impregnation step and, to some extent, of the whole preparative sequence. This study is very relevant to the synthesis of submonolayer/monolayer nickel catalysts supported on TiO(2) following equilibrium deposition filtration (otherwise called equilibrium adsorption).

Interfacial coordination chemistry for catalyst preparation

“…Langmuir dependence with well-defined saturation coverages. A complementary approach of the coverage-dependent chemistry of ISC, based on adsorption isotherms and UV-visible spectroscopy, was proposed at the same period by the Patras group for the optimization of the preparation of nickel and cobalt catalysts on alumina and titania [78][79][80].…”

Structure of Co(II) Species Formed on the Surface of γ-Alumina Upon Interfacial Deposition

“…Specifically, we follow the evolution of the various Co(II) species formed at the interface as the surface concentration of the Co(II) increases. The study is part of the entire efforts of our group to rationalize the method of preparation of supported catalysts and shift catalyst preparation from the art to the science [24][25][26][27][28][29][30][31][32].…”

Advanced Synthesis and Characterization of Vanadia/Titania Catalysts through a Molecular Approach

“…The last can be achieved by working at too low V concentrations and a very high solid/solution ratio [41,49,64]. A successful fitting of these curves is only possible if the right deposition mechanism is adopted in the simulation [49][50][51][52][53]. This is the case in Figure 4.…”

“…Moreover, excellent reviews and book chapters have been published on this subject, showing how the more sophisticated synthesis methods of supported catalysts can help us rationalize their synthesis, improve their efficiency, and control the formation of their active sites [41][42][43][44][45][46][47][48]. In this context, in the last years, we have developed supported transition metal catalysts through a molecular approach, using the equilibrium deposition filtration method (EDF), in order to fully control the impregnation step and specify the transition metal ionic species deposited on the support surface [49][50][51][52][53]. The present study is concerned with an effort to follow the formation of the vanadia site from the first to the final step of the catalyst synthesis procedure using the EDF method.…”